The present invention relates to ion implantation or ion bombardment apparatus and more particularly to such apparatus including means for effectively retaining selected subatmospheric pressures, i.e., vacuum levels in the system. In ion implantation apparatus where wafers are being loaded into and out of target chambers positioned with respect to the beam axis, it is important that pump-down times, i.e., times necessary to restore the apparatus to its operational vacuum be as short as possible and that the pump-down be carried out as effectively as possible. In order to achieve such short pump-down times, conventional ion implantation apparatus utilizes a plurality of vacuum pumps along the beam line and target chamber. By beam line and target chamber we are referring to that portion of the ion implantation apparatus including or beyond the mass separator or analyzing magnet wherein the ions selected to bombard the target are provided with a trajectory along the beam axis. A plurality of vacuum pumps are less than completely desirable from cost, reliability and packaging viewpoints.
While it would be highly desirable to reduce the number of vacuum pumps along the beam line, present ion implantation systems have found this goal to be a problem because this portion of the apparatus utilizes beam defining structures with very small apertures, thereby severely reducing the gas conductance of this portion toward a single pump.